Added description of the NPTL Abort issue.

documentation/KnownProblems.html

@@ -14,6 +14,16 @@
 <ul>
   <li>It is not possible to build this version of base on HP-UX due to C++
     issues which appear to be bugs in the HP-UX compiler.</li>
+  <li>When shutting down any CA client or IOC on a recent version of GNU/Linux
+    (one that includes the NPTL threading library) the following message may
+    appear, possibly preceeded by a message about a thread terminating due to a
+    C++ exception:
+    <blockquote><pre>FATAL: exception not rethrown</pre></blockquote>
+    An <tt>Abort</tt> message and a core-dump may also be generated if the
+    user's settings allow this. The error is caused by the implementation of the
+    pthread_cancel() mechanism in NPTL, which uses the C++ exception mechanism
+    without providing any means to distinguish the cancel exception to regular
+    C++ exception handlers.</li>
 </ul>
 
 </body>